Re: [PATCH] mm: allow a controlled amount of unfairness in the page lock

[Greg KH]

On Wed, Aug 23, 2023 at 06:16:42AM +0000, Maximilian Heyne wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> 
> [ upstream commit 5ef64cc8987a9211d3f3667331ba3411a94ddc79 ]
> 
> Commit 2a9127fcf229 ("mm: rewrite wait_on_page_bit_common() logic") made
> the page locking entirely fair, in that if a waiter came in while the
> lock was held, the lock would be transferred to the lockers strictly in
> order.
> 
> That was intended to finally get rid of the long-reported watchdog
> failures that involved the page lock under extreme load, where a process
> could end up waiting essentially forever, as other page lockers stole
> the lock from under it.
> 
> It also improved some benchmarks, but it ended up causing huge
> performance regressions on others, simply because fair lock behavior
> doesn't end up giving out the lock as aggressively, causing better
> worst-case latency, but potentially much worse average latencies and
> throughput.
> 
> Instead of reverting that change entirely, this introduces a controlled
> amount of unfairness, with a sysctl knob to tune it if somebody needs
> to.  But the default value should hopefully be good for any normal load,
> allowing a few rounds of lock stealing, but enforcing the strict
> ordering before the lock has been stolen too many times.
> 
> There is also a hint from Matthieu Baerts that the fair page coloring
> may end up exposing an ABBA deadlock that is hidden by the usual
> optimistic lock stealing, and while the unfairness doesn't fix the
> fundamental issue (and I'm still looking at that), it avoids it in
> practice.
> 
> The amount of unfairness can be modified by writing a new value to the
> 'sysctl_page_lock_unfairness' variable (default value of 5, exposed
> through /proc/sys/vm/page_lock_unfairness), but that is hopefully
> something we'd use mainly for debugging rather than being necessary for
> any deep system tuning.
> 
> This whole issue has exposed just how critical the page lock can be, and
> how contended it gets under certain locks.  And the main contention
> doesn't really seem to be anything related to IO (which was the origin
> of this lock), but for things like just verifying that the page file
> mapping is stable while faulting in the page into a page table.
> 
> Link: https://lore.kernel.org/linux-fsdevel/ed8442fd-6f54-dd84-cd4a-941e8b7ee603@xxxxxxxxxxxxxxxxxx/
> Link: https://www.phoronix.com/scan.php?page=article&item=linux-50-59&num=1
> Link: https://lore.kernel.org/linux-fsdevel/c560a38d-8313-51fb-b1ec-e904bd8836bc@xxxxxxxxxxxx/
> Reported-and-tested-by: Michael Larabel <Michael@xxxxxxxxxxxxxxxxxx>
> Tested-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Chris Mason <clm@xxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: <stable@xxxxxxxxxxxxxxx> # 5.4
> [ mheyne: fixed contextual conflict in mm/filemap.c due to missing
>   commit c7510ab2cf5c ("mm: abstract out wake_page_match() from
>   wake_page_function()"). Added WQ_FLAG_CUSTOM due to missing commit
>   7f26482a872c ("locking/percpu-rwsem: Remove the embedded rwsem") ]
> Signed-off-by: Maximilian Heyne <mheyne@xxxxxxxxx>
> ---
>  include/linux/mm.h   |   2 +
>  include/linux/wait.h |   2 +
>  kernel/sysctl.c      |   8 +++
>  mm/filemap.c         | 160 ++++++++++++++++++++++++++++++++++---------
>  4 files changed, 141 insertions(+), 31 deletions(-)

This was also backported here:
	https://lore.kernel.org/r/20230821222547.483583-1-saeed.mirzamohammadi@xxxxxxxxxx
before yours.

I took that one, can you verify that it is identical to yours and works
properly as well?

thanks,

greg k-h